1. Field of the Invention
The present invention relates to polyolefin films having high melting points combined with improved flexibility, high puncture and tear resistance, and good contact clarity. The monolayer or multilayer films are produced using extrusion methods, either blown film extrusion, cast film extrusion, or lamination, and comprise a polyolefin which is a copolymer of propylene and ethylene.
2. Detailed Description of the Invention
Polyolefins are widely used for the production of films. These films can be produced by either blown extrusion or by slot-die extrusion with subsequent cooling using a chill roll or water bath and a variety of products with a range of properties are available.
The high melt point and highly crystalline structure of polypropylene make it well suited for applications where stiffer and strong films are required. The narrow melting range and high rate of crystallization do, however, create processing problems. To some extent these processing difficulties have been overcome by blending or by copolymerization but not without compromising optical properties, tensile strength, and other important film characteristics.
In a discussion of propylene polymers, W. Schoene in his article entitled "PP Specialty Film Products for Film Applications" (Specialty Plastics Conference 89, Zurich, 1989) divided propylene polymers into four classes--highly crystalline homopolymers, homopolymers with reduced crystallinity, random copolymers and impact copolymers. From the general properties listed by Schoene for the two copolymer types, it is apparent that clarity and stiffness are sacrificed in order to improve in the low temperature impact resistance.
Resins in the polyethylene homopolymer and copolymer families provide a broad spectrum of film products with their good optics, heat seal and processability. While these films are well suited for many uses, the low melting point of the resins precludes their use in certain applications.
It would be advantageous if polymers suitable for films were available which combine the advantages of both polypropylene and polyethylene resulting in films that display good contact clarity, sealing characteristics and softness with high impact and tear resistance. It would be even more desirable if these products had melting points substantially higher than that of low density polyethylene (LDPE) and preferably as high or higher than high density polyethylene (HDPE).
These and other advantages are achieved with the products of the instant invention which are described in more detail to follow.